Add support for WEBP screenshots

[DandelionSprout]

At the moment, the screenshot section of the Reports form only list PNG and JP(E?)G as alternatives, so I think WEBP should be added too.

My motivation is that I've had a beef with the online anti-WEBP cult for 3 or 4 years now, as the cult that consist of tens of millions of people online hate WEBP for either made-up or circular reasons; among them that they think WEBP "isn't used by any software" (PaintNET, Discord, and XnViewMP can attest to that it is being used).

I myself am very pro-WEBP after I studied the format's image compression sizing, and concluded a lossless WEBP was 15% smaller in filesize than OptiPNG.

While screenshots of websites and computer windows in WEBP remain almost non-existent (mostly due to Windows' image clipboarding defaulting to PNG, even when the clipped image was JPG), I feel that adding WEBP image support would be a very small but appreciated double-fix for the cult's claims that "WEBP isn't supported by anything" and "WEBP must be destroyed in hellfire".